Not applicable.
Not applicable.
The present invention relates generally to optically variable pigments and more particularly to all-dielectric diffractive pigments, including all-dielectric diffractive pigments with thin-film interference structures.
Optically variable pigments (xe2x80x9cOVPsxe2x80x9d(trademark)) are used in a wide variety of applications. They can be used in paint or ink, or mixed with plastic. Such paint or ink is used for decorative purposes or as an anti-counterfeiting measure on currency. Although the pigment flakes might be very small, optical effects arise from the aggregate effects of the flakes, which are often generally aligned with the plane of the substrate. One type of OVP uses thin-film layers on a substrate to form a Fabry-Perot type optical structure. Generally, a layer of optically absorbing material is separated from a reflective layer by a dielectric (spacer) layer. Additional layers may be added for additional effects, such as adding additional spacer-absorber layer pairs. The reflective layer is often a layer of metal, making the pigment flake opaque. Images printed or painted with the opaque pigment flakes are also opaque, or the pigment flakes dull or change the underlying color.
Transparent pigment flakes can be made using alternating layers of dielectric materials in a (high-low-high)n or (low-high-low)n fashion to form an optical interference stack, which is often referred to as a dichroic stack. Dichroic pigment flakes can be essentially transparent, reflecting one color and transmitting another color. The color of an image printed with some dichroic pigment flakes changes with viewing angle, and such pigments allow overprinting, so that the viewer can see the underlying image through the dichroic pigment flakes.
Another type of pigment uses a diffraction pattern (grating), such as a series of grooves, to create a diffractive interference structure. The diffraction grating is formed in a reflective layer in the flake, similar to the patterns formed in diffractive foils. Diffractive pigments have been used to create an iridescent effect, both on printed media and in paints, such as automotive paint. Unfortunately, the reflective layers are usually opaque, thus the diffractive pigment flakes dulls or changes the underlying color, as with the Fabry-Perot-type opaque pigment flakes.
Pearlescent paints and pearlescent additives have been popular for some time for painting cars, motorcycles, boats, helmets, and other objects. Many such pigments are made using processed mica flake that is coated with a high-index material, such as iron oxide or titania. The thickness of the mica and/or coating can be varied to obtain different pearlescent colors; however, paints made with such pigments do not show a diffractive effect.
A pigment flake produced according to embodiments of the present invention provide diffractive effects without a metal reflective layer. In a particular embodiment, a diffractive pigment flake has a diffraction grating formed on the surface of an inorganic dielectric thin film layer. The surface provides a diffractive interface when the pigment flake is dispersed in a carrier, such as an ink vehicle or a paint vehicle, or when a second thin film layer is formed over the diffraction grating pattern. In some embodiments, the diffraction grating pattern is replicated through the second, and perhaps subsequent, thin film layer(s), and additional diffractive interfaces are formed. Strong diffractive effects are enabled when the pigment flakes with five or less dielectric layers are dispersed in a carrier.
Unlike diffractive flakes having opaque metal reflectors, some embodiments of all-dielectric diffractive pigment flakes according to the present invention are semi-transparent, and can have reflecting and/or transmitting colors to be matched to the object that they are applied on. In other embodiments, all-dielectric diffractive pigment flakes having a high-low-high-low-high optical design centered at 550 nm provides a white diffractive effect when applied over a white object. Such embodiments are alternatively applied over colored surfaces to impart a diffractive effect without substantially changing the background color of the surface. In some embodiments of all-dielectric diffractive pigment flakes made according to embodiments of the present invention, the thickness of the dielectric layers is chosen to create thin-film interference, and impart a color-shifting or pearlescent effect, as well as a diffractive effect.